Unidirectional torque couplings transmit torques in only one rotational direction, and thus do not transmit (or transmit significantly less torque) in the opposite rotational direction, thereby providing a so called “freewheel”. Such apparatuses are typically used in drive technology, such as those with non permanently driven two-wheeled vehicles. While in the drive phase, the drive force, which is provided by means of muscles or a motor, is transmitted by use of a coupling from the drive to the wheel; the wheel being able to rotate freely in phases without (or without sufficient) drive, and the wheel being decoupled from the still standing or only slower rotating drive (“coasting”).
In two wheel vehicle technology, such apparatuses are often simply called a “freewheel”. In bicycles, they are typically mounted in the rear wheel hub. They allow transmitting drive forces provided from a foot pedal over the chain and onto the hub, as long as the angular velocity of the rear sprocket is equal to the angular velocity of the hub (and therefore, of the rear wheel). This equality is achieved in the drive direction by a mechanical, form-closed and/or frictional coupling between rear sprocket and hub. If, on the other hand, the angular velocity of the rear sprocket is slower than the angular velocity of the hub, the hub can rotate freely without being braked by the sprocket or further drive train components which are connected to it, respectively.
For the transmission of torque from the rear sprocket onto the hub, spur-cut freewheel disks are used. These have a saw tooth profile and are pressed against each other by means of a spring or a magnetic force. One of the freewheel disks is connected to the drive (rear sprocket), and the other is connected to the output (hub). In the drive direction, both freewheel disks interlock, such that the torque can be transmitted from one disk to the other, and thus, from the drive to the output. Contrary to the drive direction, one of the freewheel disks, which is axially moveable, is pressed away by the saw tooth profile from the other free wheel disk such that it slips, thereby preventing the continued transmission of torque. Often, this slipping is associated with an acoustically well perceptible clicking.
The axially moveable freewheel disk is often equipped with a spline profile at its circumferential external surface. It's gears form-lock with an accordingly designed toothing of the inside of the hub body. A tilting and thus cocking of the moveable freewheel disk must be avoided, just as the introduction of particles onto the spline profile, since this could otherwise result in a jamming of the free wheel disk. Thus, the repeated axial movement produces abrasion in the spline profile which on one hand continuously increases the clearance, and which can contribute to a contamination caused failure on the other. Further, the safe operation is also influenced by an ingress of dirt from the outside, as well as by the temperature variable viscosity of the imperatively necessary lubricant.
The contact force of both tooth-locking disks is exclusively provided by the above mentioned springs and/or by magnetic force. If it is too high, this results in an unnecessary high friction and thus an increased abrasion of the slipping freewheel, together with an often unpleasantly loud noise emission. If it is too weak, the result can be an undesired slipping of the toothing in the drive direction. Because of the continuous aging of the freewheel, it can suddenly slip in the drive direction and be rendered useless after only a few years of normal operation. Therefore, injuries of the driver due to such a defect are not unlikely. In order to minimize the danger of such a defect, the saw tooth profile is manufactured with a slight undercut which is complicated to produce, so that the toothings pull themselves against each other when the coupling is in the drive phase.